A hearing aid usually utilizes the basic components shown in the device 10 in FIG. 1 of the drawings. A microphone 11 senses ambient sound 12 and develops an electrical signal representative of that sound. The electrical signal is amplified, in an amplifier 13, and then used to drive a sound reproducer or transducer 14, frequently called a receiver. The receiver 14 may be coupled to the ear canal 15 of the user of the hearing aid by a sound transmission tube 17, supplying a sonic signal 16 to the hearing impaired person using the aid 10. The entire device 10, including components not shown in FIG. 1 (e.g., an on-off-switch, a battery, a volume control, etc.) is often small enough to fit in the user's ear, though other packaging arrangements have been and are used.
The hearing losses of a major portion of the hearing-impaired population occur primarily in the higher frequency end of the audio spectrum. These people frequently have normal or near normal hearing at the lower and middle frequencies. Thus, hearing aids tend to be designed to emphasize amplification of the higher audio frequencies. They may provide little if any amplification at the lower end of the audio spectrum.
One popular approach is to provide a vent or channel in the ear mold or through the hearing aid itself, if it is of the in-the-ear variety. That channel is apportioned so that low frequency sounds can enter the ear directly, without amplification, while high frequency sounds that are amplified are retained within the ear by frequency-discriminating characteristics of this vent. These effects may be reinforced by the design of amplifier 13 and microphone 11. Especially designed microphones are produced for this purpose, which are most sensitive at the higher frequencies; see curve A in FIG. 2.
Historically, little if any means have been found to effectuate use of the frequency characteristics of the receiver (earphone) itself to aid in this frequency selectivity. There have been older and larger versions of receivers made and sold that mimic the method used to obtain the frequency characteristic in microphones of the type indicated in curves B and C in FIG. 2. This may be accomplished in a microphone by providing a vent or tube leading from one side of the diaphragm to the other, thus allowing the sound pressure to equalize at low frequencies. There are several difficulties with this approach in the modern, more miniaturized receiver; a major problem has been to find enough space for an acoustically adequate vent. Also, probably because of the way a receiver is coupled to the ear cavity, there is a considerable loss in sensitivity using this approach.
While there is no consensus on the matter, one school of thought believes that a high frequency pass band of about an octave starting at about 3000 Hz (2500 to 3500 Hz) will be beneficial.
A conventional hearing aid receiver presently consists of an electromagnetic motor mechanism which operates a diaphragm. The air displaced by this diaphragm, on one side, is channeled through a tube into the ear canal, creating the desired sound. The air displaced on the other side is usually compacted in the volume enclosed by the receiver housing. When connected to an occluded (unvented) ear canal or to a test chamber, usually known as a coupler, this mechanism produces a frequency characteristic of the type shown as curve W in FIG. 3. The principle components controlling the frequency of the initial resonance peak 21 are the mechanical system of the motor and the channel or tube leading the sound from the diaphragm into the ear (receiver 14 and tube 17 in FIG. 1). The second resonance 22 of curve W is controlled by the necessary volume of air within the receiver that collects the sound off of the diaphragm, the channel or tube that conducts this sound to the ear canal, and the remaining portion of the ear canal.